1. Technical Field
The present invention generally relates to a liquid ejecting apparatus such as an ink-jet printer and a method for controlling the liquid ejecting apparatus. In particular, the invention relates to a liquid ejecting apparatus that can control the ejection of liquid by applying an ejection pulse to pressure generators and a method for controlling such a liquid ejecting apparatus.
2. Related Art
A liquid ejecting apparatus is an apparatus that can eject various kinds of liquid. The apparatus is provided with a head for liquid ejection. A plurality of nozzles from which the liquid is ejected is formed in the liquid ejecting head. An example of a liquid ejecting apparatus is an image recording apparatus such as an ink-jet printer. An ink-jet printer is provided with an ink-jet recording head, which is an example of various kinds of a liquid ejecting head. An ink-jet printer performs recording by ejecting ink in the form of ink droplets from nozzles of the ink-jet recording head toward a recording target medium such as a sheet of printing paper. As a result of the landing of the discharged ink droplets on the surface of the target object, that is, the recording target medium, dots are formed thereon. In this way, the ink-jet printer records an image and the like on the recording target medium. The ink-jet recording head may be hereinafter simply referred to as a “recording head”. The ink-jet printer may be hereinafter simply referred to as a “printer”. These days, the application of such a liquid ejecting apparatus is not limited to an image recording apparatus mentioned above; for example, a liquid ejecting apparatus is used as, among many types of manufacturing apparatuses, manufacturing equipment used for production of a color filter for a liquid crystal display device.
For example, the printer has a plurality of nozzle lines each of which is formed as a group of nozzles that are aligned. A pressure chamber is formed in communication with each of the nozzles. An ejection pulse is applied to a means for generating pressure. When the pressure generator is driven, pressure fluctuation occurs in liquid that is retained in the corresponding pressure chamber. The pressure fluctuation is utilized to eject the liquid from the corresponding nozzle, which is in communication with the pressure chamber. Some printers use piezoelectric vibration elements as the pressure generators. Such a printer operates as follows. As a preparatory step, a pressure chamber is expanded first (an expansion step). Then, the pressure chamber is kept in the expanded state for a predetermined length of time (a hold step). Next, the pressure chamber is contracted rapidly (a contraction step). As the pressure chamber contracts, ink that is retained in the pressure chamber is pressurized. By this means, the printer discharges a very small ink droplet of several nanograms (ng) to more than ten nanograms from the nozzle. An example of such a printer of the related art is disclosed in JP-A-2001-63042. According to the printer disclosed in JP-A-2001-63042, parameters such as the waveform of a driving signal used for driving the piezoelectric vibration element, the voltage thereof, and the like are set with due consideration given to the natural frequency of a pressure wave of ink retained in the pressure chamber. Therefore, the disclosed printer is capable of discharging a very small ink droplet without lowering the level of reliability and durability of the piezoelectric actuator.
In the operation of the above type of a printer, when ink is ejected from a nozzle, the tail part of an ink droplet sometimes comes off to form into a satellite droplet, which follows a main droplet. Normally, the velocity of the movement of a satellite droplet in the air is lower than the velocity of the movement of a main droplet in the air. For this reason, the length of time from ejection from a nozzle to landing on a recording target medium for a main droplet is different from that for a satellite droplet. Therefore, when printing is performed while moving a recording head and a recording target medium relative to each other, a position at which a main droplet lands on the recording target medium and a position at which a satellite droplet lands on the recording target medium are shifted from each other. The shift between the landing position of the main droplet and the landing position of the satellite droplet causes degradation in the quality of a print image, which is a problem that remains to be solved.